So, one of my players is riding on a flying carpet, wearing metal armor, in a thunderstorm.
What chance should I assign for him being hit by lightning? He's got roughly 25 miles to cover.
My gut feeling is that the answer should just be "yes" every 5 or 6 miles covered. However it turns out that my gut feeling is probably wrong. Doing a little research, and it seems that the average airliner is struck by lightning only once a year. Given that they do at times fly through thunderstorms it seems like a low occurrence. Based on that I'd say make it a 1 in 100 roll for each trip through a thunderstorm.
http://www.livescience.com/32638-do-planes-get-struck-by-lightning.html
Alternatively, you could skip the 1 in 100 row and declare that flying through a storm leaves him and his equipment completely soaked, and he might suffer a -1 penalty until he rests for 1 turn.
NASA flew a specially protected F-106 into 1,400 thunderstorms in the 1980s. It was struck half the time. I'd put the odds somewhat lower for a person on a flying carpet, since it's a smaller amount of metal, but 1-in-4 or 1-in-6 sound fair.